It is generally recognized that there is a need for electrically powered expansion and contraction devices in many fields including robotics and prosthetics. Expansion/contraction units, or "motive" units, are needed in welding robots on automobile assembly lines, for example, and in undersea robots useful for oil rig repair.
Just as robot "limbs" need motive units, artificial or prosthetic limbs for humans also require motive units. Similarly, artificial hearts require some type of motive unit to pump blood.
Robot and prosthetic limbs aside, there are innumerable applications where simple expansion/contraction devices are needed. The remainder of the present application will be directed to artificial limbs, though it should again be noted that the expansion/contraction device of the present invention is not limited to motive components for artificial limbs. The expansion/contraction device of the present invention will variously be referred to as an "artificial muscle," "synthetic muscle," motive unit or expansion and contraction device throughout the application.
Several types of motive units for electrically powered artificial muscles have been developed. Electrically powered rotary motors, including stepper motors, sometimes in combination with simple solenoids, have been utilized. For example, see U.S. Pat. Nos. 4,074,367 and 4,067,070. Another type of artificial muscle that has been developed comprises an electromagnet imbedded in a resilient material that acts to bind a plurality of magnetic particles. U.S. Pat. No. 2,532,876, issued to Asche et al, and U.S. Pat. No. 4,176,411, issued to Runge, are representative of this type of synthetic muscle. The Asche and Runge devices function in a similar fashion: When the electromagnet that is surrounded by or imbedded in the resilient material is energized, either directly or inductively, the magnetic particles imbedded in the resilient material are either pulled toward the electromagnet or pushed outward, away from the electromagnet, depending on whether the magnetic particles are themselves magnetized, and depending on the orientation of the poles of the electromagnet as compared to the orientation of the poles of the individual magnetic particles. It should be noted that throughout the present application the term "magnetic" connotes something that is magnetized, capable of being magnetized, or simply attracted and/or repelled by a force created by a magnetic field in the proximity of the material. "Magnetized" in the present application, unless the context indicates otherwise, means that the magnetic material has an established north and south pole. A "magnet" in the present application may be either a permanent magnet or an electromagnet.
It is perceived that the Asche et al and Runge type of synthetic muscle possesses several shortcomings. First, a certain amount of energy is irreversibly wasted each time the resilient substrate is deformed and then returned to its original shape. This is due to the nature of the resilient material that comprises the substrate. Secondly, it is perceived that the amount of contraction or expansion of an Asche or Runge muscle, or similar device, is dependent to some degree on the load on the synthetic muscle. That is, once the electromagnet of the Asche or Runge muscle is stimulated, the magentic particles will move and the substrate will deform until forces generated by the resilient substrate and the load on the muscle combine to substantially equal the force generated by the electromagnet on the magnetic particles, and an equilibrium point is established. It can be seen that when the load on the synthetic muscle changes, this point of equilibrium changes and the amount of expansion or contraction varies. Such a muscle therefore does not operate in an "all-or-none" mode. Natural muscle fibers, on the other hand, either fully contract or do not contract at all, thereby functioning as a binary (contracted or not contracted) system. It is thought that a synthetic "muscle fiber" functioning in the nature of a natural muscle fiber, i.e., according to the all-or-none law, is desirable. A "binary" or all-or-none muscle fiber is more amenable to digital control than a muscle of the Asche or Runge design, and perhaps ultimately would be more easily interfaced and controlled by the impulses transmitted by the nervous system of an individual.
Another perceived shortcoming of the Asche/Runge type of muscle is that "muscle tone" is not easily achievable. Under a varying load, an Asche/Runge muscle would deform until a new equilibrium point is established, unless the electromagnet is energized to compensate for the varying load to maintain the original equilibrium point.
It is also perceived that resilient materials as used in the Asche and Runge synthetic muscles are typically sensitive to temperature variations and aging effects. The modulus of elasticity or spring constant of a resilient material is usually at least somewhat dependent upon the age of the material and its physical environment. Furthermore, the spring constant of a resilient material may be a function of the degree of deformation of the material, if the material has non-linear characteristics.
The invention of the present application addresses the aforementioned shortcomings possessed by the prior art synthetic muscles. A synthetic muscle according to the present invention is comprised of a plurality of motor units, each of the motor units preferably comprising a pair of electromagnets and a pair of motor elements that are movable when subjected to a magnetic field. In this embodiment, an electromagnet and a motor element form a motor subunit, and the pair of motor subunits are interconnected so that when they contract the motor unit contracts and thus the entire synthetic muscle contracts. In a preferred embodiment, expansion of the synthetic muscle follows a similar sequence. Each motor subunit of the present invention freely expands and contracts between an upper and a lower limit. For example, in the contraction mode of the synthetic muscle according to the present invention, when the electromagnet is energized, either directly or inductively, the motor element snaps from a distal position, the upper limit, to a proximal position, the lower limit. Each of the positions is limited by stop means as further discussed below.
The motor unit for an artificial limb that comprises an electromagnetic expansion/contraction device according to the present invention is able to produce a linear motion, thus better mimicking a natural muscle without requiring a rotary-to-linear transducer. An artificial muscle according to the present invention can be made quite small depending on the application. Also, rotary bearings are generally unnecessary. Thus, the presently-invented synthetic muscle motor unit addresses the shortcomings of electrically powered rotary motors. Also, a muscle according to the instant invention is contracted or expanded to a degree dependent on the number of motor units that are energized or stimulated. This characteristic closely mimics the functioning of a natural muscle and also allows the artificial muscle to be more easily controlled using digital electronic techniques. Comparable control techniques for rotary motors are more complicated.
A further advantage of the present invention over the artificial muscles of Asche and Runge is that a resilient substrate is not required to bind a plurality of magnetic particles, and therefore the problems of a varying spring constant and dissipation of energy during a contraction/expansion cycle are obviated. The amount of contraction/expansion of the presently-invented artificial muscle is dependent upon the number of motor units energized, whereas the amount of deformation of an Asche or Runge muscle depends on the magnitude of the load and the amount of current driven through the electromagnet's coil. Thus, again, the all-or-none law is more closely observed with the artificial muscle of the present invention, readily adapting to digital control.
Also, the movement resolution of an artificial muscle according to the present invention depends on the number of motor units in end-to-end or collinear alignment, the larger the number of units the finer the resolution. The number of motor units that is energized can be gradually changed to effect a smooth contraction or expansion of the artificial muscle. Conversely, in the Asche or Runge muscle, if the electromagnet is pulsed with a predetermined amount of current the muscle will deform accordingly and fairly suddenly, leading to a contraction or expansion that is less smooth.
With respect to all of the artificial muscles of the prior art discussed above, an artificial muscle of the present application better addresses the problem of "muscle tonus." Muscle tone in a natural muscle is achieved by the continuous stimulation of a select number of muscle fibers. In the instant invention, the same or similar characteristic is achieved by energizing a select number of motor units.